The invention relates to the field of watermarking an information signal and detecting a watermark.
Watermarks are perceptually invisible messages embedded in information signals such as multimedia material, e.g. audio, still pictures, animations or video. Watermarks can be used to identify the copyright ownership of information. They allow a copyright owner to trace illegal copies of his material by inspecting whether his watermark is present in said copies.
Watermarks are embedded in an information signal by modifying data samples of the signal (e.g. audio samples of an audio signal, pixels of an image, transform coefficients of a transform-coded signal, etc.) such that the original is not perceptibly affected. Various methods of watermarking are known in the art. For example, pixels of an original image are slightly incremented or decremented in accordance with corresponding bits of a binary watermark pattern.
In order to detect whether an information signal has an embedded watermark, the signal is subjected to a statistical analysis. The statistical analysis yields a parameter, hereinafter referred to as xe2x80x9cdecision variablexe2x80x9d, which indicates to which extent the watermark is present in the signal. For example, if an image signal is watermarked by incrementing or decrementing its pixels in accordance with a watermark pattern, the decision variable may be the amount of correlation between the signal and an applied reference copy of the watermark. If an image is watermarked by modifying selected pixels, a prediction for said pixels is calculated from temporally or spatially adjacent pixels. The decision variable may then be the number of pixels being sufficiently different from their prediction.
Watermark detectors generate a binary output signal indicating xe2x80x9cwatermark foundxe2x80x9d or xe2x80x9cno watermark foundxe2x80x9d. That is achieved by comparing the decision variable with a predetermined threshold. If the value of the decision variable exceeds the threshold, the watermark is considered to be present in the signal. in which I is the total number of pixels.
The correlation value y is applied to a comparator 23 for comparison with a threshold value Ythr. The comparator produces an output D=1 (watermark found) for Y greater than Ythr and an output D=0 (no watermark found) for Y less than Ythr. The watermark pattern w and the threshold value Ythr are carefully chosen to avoid that the detector makes too often a false decision.
FIG. 3 shows a system comprising a watermark embedder and detector in accordance with the invention. The same reference numerals are used for circuit elements which are correspond with elements which are already been shown in FIG. 1. In this example, the applied watermark w is assumed to have a 16 multi-bit values. Accordingly, the input image is divided into 16 blocks (n=16) by block-division means 11. By further block-division means 12, the watermark pattern also is divided into 16 subpatterns. A multi-bit value w1 . . . wn controls whether or not a subpattern is added to the image. This is illustrated in FIG. 4 in which the blank rectangles indicate that the corresponding image blocks are not watermarked. To compensate for the reduced watermark xe2x80x98energyxe2x80x99 caused by not watermarking all image blocks, other blocks are watermarked more intensively, for example by incrementing/decrementing the respective pixels by the value 2 instead of 1 as discussed before.
The watermark detector 2 now comprises partial accumulators 22.1, 22.2 . . . 22.n, one for each multi-bit value of the watermark w. Their outputs are combined in a final adding stage 24 to obtain the decision variable y which is applied to the comparator 23 which produces an output D indication of whether a watermark is or is not present, as described above. The individual outputs of the partial accumulators represent individual correlation values y1 . . . yn. After normalization, they have a value between 0 (no correlation) and 1 (maximum amount of correlation). A subsequent evaluation circuit 25 calculates, for each correlation value, its difference relative to the corresponding one of the multi-bit watermark values to be checked and calculates the   M  =            ∑      n        ⁢          xe2x80x83        ⁢                  {                              w            ⁢                          (              n              )                                -                      y            ⁢                          (              n              )                                      }            2      
sum of the squares of said differences. For example, if the watermark to be checked is a 4-bits value 1001, and the corresponding correlation values are y1=0.8, Y2=0.3, Y3=0.1 and y4=0.6, the evaluation circuit calculates
M=(0.2)2+(0.3)2+(0.1)2+(0.4)2.
The result is an indication of to what extent the detected watermark resembles the given multi-bit value.
In summary, a method and an arrangement for detecting a multi-bit watermark in a signal (e.g. a video image) are disclosed. Each value of the multi-bit watermark values is represented by an embedded watermark subpattern in the signal. Instead of detecting in a conventional
It is an object of the invention to provide a method and arrangement for detecting the presence of a multi-bit watermark value in the signal. It is also an object of the invention to provide a method and arrangement for accommodating such a multi-bit watermark value in images of a video signal.
This is achieved by a method in witch each of said multi-bit watermark values is represented by a respective watermark subpattern. The method comprising the steps of: determining, for each watermark subpattern, a correlation value indicating the extent to which said subpattern is present in the signal; combining said correlation values to detect whether the watermark is present in the signal irrespective of its multi-bit value; and comparing said correlation values and the corresponding bits of an applied multi-bit value to obtain an indication of to which extent the detected watermark has said applied multi-bit value.
The invention is based on the need to embed a multi-bit watermark in a signal. This is achieved by representing each multi-bit value of the watermark by a respective watermark subpattern. A straightforward method of detecting the watermark would include using a conventional watermark detector for each individual bit of the watermark by detecting the presence of the corresponding subpattern. However, as already mentioned above, detecting a watermark is a statistical analysis process which may result in false decisions. The reliability of each individual detection is weak because its signal strength is 1/nth (n is the number of bits) of the energy in the full watermark.
The inventors have recognized that is advantageous to detect the presence of the watermark irrespective of its multi-bit value at maximum reliability, and compare the (Hamming) distance between the intermediate results (i.e. the extents to which individual subpatterns are present in the signal) and the multi-bit value which is to be checked. The reliability of the multi-bit detection is now comparable to the reliability of the detection of an entire, full strength watermark.
The invention further provides a method of embedding a multi-bit watermark in a video image by adding a watermark pattern to said image, comprising the steps of: dividing the video image and said watermark pattern into blocks, the blocks of the watermark pattern constituting subpatterns; and representing each bit of the multi-bit value by adding or not adding a subpattern to the corresponding image block.